In the field of conveying bulk materials by endless-belt conveyors, it is desirable to have as few separate flights as possible making up a conveying system, for reasons of capital and operating cost as well as reliability. This is especially the case for conveyors that run in tunnels from one level of an underground mine to the surface. In such conveyors, transfer stations represent very substantial capital and operating costs, as well as the locations of highest operational risk. The excavation, power, access, and ventilation costs are often multiples of those in a surface drive or transfer station.
A key limitation on the length or lift that can be achieved with a single conveyor flight is the tensile strength of the conveyor belt. On long overland conveyors, the accumulation of frictional losses together with the forces required to either elevate or lower the load eventually builds to a point where the tension in the conveyor belt reaches a maximum allowable level for the belt's tension-carrying members, dictating the limit on the length of the conveyor. On conveyors that run on a substantial incline, the forces required to hold the belt and its load on the slope are the dominant forces that determine what distance of slope the conveyor can traverse before the tensile capacity of the belt is exceeded.
Further, in many of the major slope-conveyor projects to date, the flight lengths have been limited by the tensile strength of the available steel-cord conveyor belts. The usable strength of these belts is in turn limited by the fatigue strength of the splices between the dozens of discrete belt lengths that typically make up one endless belt. As the static strength of a steel cable belt increases, the fatigue strength of the splice (as a percentage of the static strength) decreases. So with current splicing technology, there is an inherent technical limit to the usable strength of steel cord belts. Therefore many major slope-belt projects have been designed with multiple conveyor flights, each flight utilizing the highest-strength steel-cord belt offered by the leading belt manufacturers. These flight length limitations imposed by belt strength have existed for as long as slope conveyor have been built, which is for roughly the last century.
Turning to solutions for the flight length limitations, it has been axiomatic in conveyor engineering that lower capital and operating costs are achieved when the required duty is met by selecting a smaller number of high-capacity components, rather than a larger number of lower-capacity components. So, for example, using two high-capacity drive trains would usually be more attractive than employing three lower-capacity drive trains. Similarly, a single conveyor that can handle 10,000 tons per hour is economically more attractive than two parallel conveyors that can handle 5,000 tons per hour each.
Another possible approach to increase the maximum achievable length of single conveyor flights is to provide discrete, relatively short belt-on-belt booster drives intermediate the head and tail pulleys of a conveyor in the form of secondary or internal belt conveyors that frictionally engage the underside of the main or carry belt. This type of arrangement is shown in FIG. 1. FIG. 2 shows a tension plot for the carry belt 102 of the conveyor system 100 of FIG. 1, where the tension in the carry belt 102 falls as the carry belt 102 passes over each booster section or booster drive 104. In practice, the length of each internal belt 106 is kept as short as possible so as not to incur excessive cost due to the duplication of belting. As such, the length of each booster drive 104 comprises only a small fraction of the overall length of the main conveyor 100. The length of the tension-transfer segments 108 shown in FIG. 2 would be much shorter and steeper in practice than suggested by FIG. 2.
The arrangement shown in FIG. 1 suffers from serious or fatal disadvantages. Excessive slack belt can be introduced by the booster section 104 over-driving the carry belt 102, which has led to catastrophic failures on long overland conveyors. In addition, it is known in the field that belt-on-belt drives can reliably transfer no more than one horsepower per longitudinal foot of belt-on-belt drive, which has made it counterintuitive to try and apply belt-on-belt drives to slope conveyors as the slope portions consume high rates of power. Furthermore, each booster unit 104, situated remotely from the main conveyor's head or tail locations 110, 112, requires a supply of power and a set of ancillary infrastructure, which poses challenges for inspection, maintenance and safety practices and adds substantially to the capital and operating costs of the conveyor system.
Another arrangement for applying belt-on-belt friction drives is shown in FIG. 3. However, this arrangement is used to separate the wearing elements of the conveyor system from the tension-carrying elements. The upper “carry” belt 202, which has a relatively low level of tensile capacity, is optimized to economically absorb the wear and impact involved in receiving and carrying the bulk material 204. The tension-carrying function is provided by the second or internal belt 206 arranged internally to the upper belt 202. The head pulley 208 of the upper belt 202 may be a non-driven pulley, or supply only a very small fraction of the total power required to drive the conveyor system 200. Almost all of the power required to drive the conveyor system 200 is applied through the pulley 210 of the inner belt 206. These types of conveyor systems do not enable the overall length of the conveyor to be any longer than a conventional single-belt system.
It is therefore desirable to provide a conveyor system, in particular an improved conveyor system implementing belt-on-belt drives, that addresses the above described problems and/or that offers improvements over existing belt-on-belt conveyor systems.